About 1 in 1000 children has hereditary hearing loss with mutations of SLC26A4 being one of the most prevalent known causes of hereditary deafness. The high incidence provides an imperative to investigate the etiology of SLC26A4-related deafness with the ultimate goal to develop strategies to restore and preserve hearing in afflicted individuals. The human gene SLC26A4 and the mouse ortholog Slc26a4 code for pendrin. Studies in mouse have demonstrated that pendrin is a Cl-/HCO3- exchanger in the cochlea, vestibular labyrinth and the endolymphatic sac. Lack of pendrin during embryonic development causes an acidification of endolymph and a mismatch between fluid secretion in the vestibular labyrinth and pendrin-dependent fluid absorption in the endolymphatic sac that leads to an enlargement of the membranous labyrinth and an impairment of cochlear development. The enlarged vestibular aqueduct, frequently associated with mutations of SLC26A4, appears to be a fossil-like record of such an enlargement that was present during embryonic development. Mechanisms of fluid secretion and fluid absorption in the embryonic inner ear are virtually unknown. Filling this gap in our understanding of inner ear development is critical toward the development of treatments to protect hearing in individuals afflicted with mutations of SLC26A4. We have developed four Specific Aims that address the most salient questions in mouse models: Aim1) - what is the ionic composition of endolymph in the embryonic cochlea and the endolymphatic sac? This aim will be addressed by measuring the composition of inner ear fluids with ion-selective electrodes. Aim2) what mechanisms mediate fluid secretion? Aim3) what mechanisms mediate fluid absorption? These aims will be addressed by testing hypothetical models of fluid secretion and fluid absorption. The onset and location of expression of candidate channels and transporters will be determined by qRT-PCR and immunocytochemistry or in-situ hybridization. Selected models will be tested in compound-mutant mice. Deficient expression of channels or transporters implicated in fluid secretion is expected to curb cochlear enlargement and possibly restore normal cochlear development in the absence of pendrin. Conversely, deficient expression of channels or transporters implicated in fluid absorption is expected to cause cochlear enlargement in the presence of pendrin. Aim4) is restoration of pendrin expression solely to the endolymphatic sac sufficient to prevent cochlear enlargement and deafness? This aim will be addressed by generating mice with pendrin expression limited to the endolymphatic sac of the inner ear. Studies will include monitoring cochlear lumen formation by histology, measurements of the endocochlear potential and endolymphatic pH with ion-selective electrodes and evaluating hearing by auditory brain stem recordings. Completion of these four Aims will establish an understanding of fluid homeostasis in cochlear development which holds the promise to reveal treatment strategies that are suitable to prevent deafness in individuals that bear mutations of SLC26A4.